The present invention relates generally to semiconductor manufacturing processes, and, more particularly, to an etch stop layer for a magnetic tunnel junction (MTJ) cap structure, and method for patterning an MTJ stack using the same.
Magnetic (or magneto-resistive) random access memory (MRAM) is a non-volatile random access memory technology that could potentially replace the dynamic random access memory (DRAM) as the standard memory for computing devices. The use of MRAM as a non-volatile RAM will eventually allow for “instant on” systems that come to life as soon as the system is turned on, thus saving the amount of time needed for a conventional PC, for example, to transfer boot data from a hard disk drive to volatile DRAM during system power up.
A magnetic memory element (also referred to as a tunneling magneto-resistive, or TMR device) includes a structure having ferromagnetic layers separated by a non-magnetic layer (tunnel barrier), and arranged into a magnetic tunnel junction (MTJ). Digital information is stored and represented in the memory element as directions of magnetization vectors in the magnetic layers. More specifically, the magnetic moment of one magnetic layer (also referred to as a reference layer) is fixed or pinned, while the magnetic moment of the other magnetic layer (also referred to as a “free” layer) may be switched between the same direction and the opposite direction with respect to the fixed magnetization direction of the reference layer. The orientations of the magnetic moment of the free layer are also known as “parallel” and “antiparallel” states, wherein a parallel state refers to the same magnetic alignment of the free and reference layers, while an antiparallel state refers to opposing magnetic alignments therebetween.
In newer MRAM devices, the free layer may be a loosely coupled, synthetic anti-ferromagnet having a pair of antiparallel ferromagnetic films sandwiched around a nonmagnetic spacer material of appropriate thickness. The reference layer may still be a single ferromagnetic film or a synthetic anti-ferromagnetic sandwich. In either case, the orientation of the free layer adjacent to the tunnel barrier with respect to the reference layer adjacent to the tunnel barrier determines the magnetic state of the tunnel junction.
Depending upon the magnetic state of the free layer (parallel or antiparallel), the magnetic memory element exhibits two different resistance values in response to a voltage applied across the tunnel junction barrier. The particular resistance of the TMR device thus reflects the magnetization state of the free layer, wherein resistance is “low” when the magnetization is parallel, and “high” when the magnetization is antiparallel. Accordingly, a detection of changes in resistance allows a MRAM device to provide information stored in the magnetic memory element (i.e., a read operation). The localized magnetic field generated by the passage of currents is used to align the free and reference layers to align either parallel or anti-parallel, thereby writing the bit.
In the processing and patterning of an MTJ stack, several different materials are used. For example, a lower metallization level is formed within an interlevel dielectric material (such as through well-known damascene processing techniques) to form conductive lines, followed by formation of the magnetic stack materials over the conductive lines. Again, the MTJ typically includes an antiferromagnetic layer such (but not limited to) PtMn and IrMn. On top of this pinning layer, a reference layer is deposited that typically consists of thin films of ferromagnetic alloys such as (but not limited to) NiFe and CoFe with nonmagnetic spacers of (but not limited to) Ru, Re, Os, Nb, Cr or alloys thereof. A tunnel layer such as Al2O3, for example, is deposited, followed by a free layer comprising thin films of ferromagnetic alloys such as (but not limited to) NiFe and CoFe. Again, the free layer may have two ferromagnetic films with non-magnetic spacers such as Ru, Re, Os, Nb, Cr or alloys thereof. Those skilled in the art also recognize that the order of the magnetic stack materials may be reversed. That is, the reference layer may be on top of the stack, and the free layer may be underneath the tunnel layer.
In any case, the resulting MTJ stack is then provided with a protective cap layer formed thereupon, typically at a thickness of about a few hundred angstroms. The cap layer is typically formed from a conductive material such as tantalum (Ta), tantalum nitride (TaN) or titanium nitride (TiN) that protects the magnetic stack materials from exposure to the ambient. Then, a hardmask layer is formed over the cap layer for subsequent opening and patterning of the MTJ stack materials. It is desirable in integration schemes involving highly scaled-down tunnel junctions to have a self-aligned, conductive stud (e.g., around 1000 angstroms thick) used to make contact with upper level bitline wiring. The self-alignment of the stud to the tunnel junction is realized by using a conductive metal as a hardmask in the etching of the tunnel junction. However, in conventional processing, the material used for the hardmask layer is the same or similar (e.g., Ta, TaN, TiN) to the material used for the cap layer. As such, there is generally poor etch selectivity of the hardmask layer with respect to the cap layer, and thus stopping the hardmask etch on the cap layer is problematic.
Accordingly, it would desirable to be able to provide a cap structure that has etch selectivity with respect to an overlying hardmask, so as to allow for a separate etch of the cap structure that will stop on the magnetic free layer of the MTJ stack and will be substantially non-corrosive thereto.